Efficient thermoelectricity in Sr 2 Nb 2 O 7 with energy-dependent relaxation times

We evaluate theoretically the thermoelectric efficiency of the layered perovskite ${\\mathrm{Sr}}_{2}{\\mathrm{Nb}}_{2}{\\mathrm{O}}_{7}$ via calculations of the electronic structure and transport coefficients within density-functional theory and Bloch-Boltzmann relaxation-time transport theory. The predicted figure-of-merit tensor $ZT$, computed with energy-, chemical potential--, and temperature-dependent relaxation times, has one component increasing monotonically from around 0.4 at room temperature to 2.4 at 1250 K at an optimal carrier density of around $2\\ifmmode\\times\\else\\texttimes\\fi{}{10}^{20}\\phantom{\\rule{0.28em}{0ex}}{\\mathrm{cm}}^{\\ensuremath{-}3}$, while the other components are small. The Seebeck coefficient is about 250 to 300 $\\ensuremath{\\mu}\\mathrm{V}$/K at optimal doping and reaches 800 $\\ensuremath{\\mu}\\mathrm{V}$/K at lower doping. We provide a python code implementing various approximations to the energy-dependent relaxation-time transport, which can be used to address different systems with an appropriate choice of material parameters.